A blue light-emitting element or an ultraviolet light-emitting element can be used as a white light source by combining it with an appropriate wavelength conversion material. Studies are being aggressively made on the application of such a white light source to, for example, backlight for liquid crystal displays and the like, light-emitting diode illumination, automotive lighting or general lighting to replace a fluorescent lamp, and some of these applications have already been put into practical use. At present, the blue light-emitting element or ultraviolet light-emitting element is produced predominantly by growing a gallium nitride-based semiconductor crystal thin film according to a metal-organic chemical vapor deposition method (MOCVD method), a molecular beam epitaxy method (MBE method) or the like technique, and these elements are collectively referred to as a gallium nitride-based light-emitting diode or a GaN-based LED.
Conventionally, most of the substrates used for the GaN-based LED are a sapphire substrate. Sapphire and GaN greatly differ in the lattice constant and therefore, a considerable number of dislocations on the order of 109/cm2 are unavoidably introduced into a GaN crystal obtained by epitaxial growth on a sapphire substrate. However, the sapphire substrate is inexpensive compared with a SiC substrate and a GaN substrate and is advantageous. Moreover, the emission efficiency of InGaN in the blue light-emitting region usually used as a quantum well layer of the GaN-based LED is not so much sensitive to the dislocation density. For this reason, the sapphire substrate is currently still a predominant substrate.
However, when the gallium nitride-based semiconductor crystal is viewed as a material for devices used in a high carrier density condition, the above-described high dislocation density incurs serious deterioration of the device characteristics. For example, in a device such as high-power LED or laser, the high dislocation density extremely reduces the device life. Also, when the active layer structure contains absolutely no In (for example, in the case of using an AlGaN layer as the active layer) or when the active layer structure contains an InGaN or InAlGaN layer having the In composition in a relatively small ratio (for example, about 0.2 or less) so as to realize light emission at a short wavelength of approximately the near ultraviolet region or less, the dislocation density dependency of the internal quantum efficiency is increased and if the dislocation density is high, the emission intensity itself decreases.
That is, in the case where the active layer structure does not contain In at all or contains an InGaN or InAlGaN layer relatively small in the In composition, demands for a low dislocation density are strict compared with a case of containing an InGaN layer having a long emission wavelength of blue or more in the active layer structure.
For achieving such a low dislocation density, it is effective to use a GaN substrate as the substrate for epitaxial growth. Use of this substrate is expected to reduce the dislocation density observed in the epitaxial layer to 108/cm2 or less, or 107/cm2 or less. Also, when the dislocation or the like of, for example, the substrate is reduced, this is expected to enable realizing a dislocation density as low as 106/cm2 or less. That is, reduction in the dislocation density by two orders or three or more orders of magnitude is expected to be achieved as compared with a case of using a sapphire substrate. For these reasons, a standing GaN substrate and a standing AlN substrate are suitable as the substrate for epitaxial growth of a gallium nitride-based semiconductor crystal.
In most of conventional attempts to epitaxially grow a gallium nitride-based semiconductor crystal on a GaN substrate that is a nitride substrate, epitaxial growth is effected on a substrate with the epitaxial growth surface being c-plane (that is, (0001) plane), in other words, on a “polar plane”. Examples of reports thereon include Patent Document 1 (JP-A-2005-347494 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)), Patent Document 2 (JP-A-2005-311072) and Patent Document 3 (JP-A-2007-67454).
Patent Document 1 discloses a technique where a nitride substrate ((0001)-plane GaN substrate) with a polar plane is used as the substrate for epitaxial growth of a GaN layer, the GaN substrate is cleaned by setting the pressure in the furnace to 30 kilopascals, a 1 μm-thick first n-type GaN buffer layer is grown at a substrate temperature of 1,050° C. while keeping the pressure in the furnace at 30 kilopascals, and after once stopping the supply of raw materials, a 1 μm-thick second n-type GaN buffer layer is further formed by heating the substrate until the substrate temperature reaches 1,100° C. while keeping the pressure in the furnace at 30 kilopascals. This crystal growth method is supposed to provide a semiconductor apparatus having a buffer layer with excellent surface flatness and good crystal quality.
Patent Document 2 discloses an invention of a light-emitting element produced by a method where after a step of removing contamination such as organic material or moisture attached to the surface of a GaN substrate under a flow of hydrogen gas, nitrogen gas and ammonia gas and simultaneously enhancing the surface crystallinity of the substrate, a multilayer structure intermediate layer composed of GaN and InGaN layers is formed on the GaN substrate while flowing nitrogen gas and hydrogen gas, and a reflective layer, an active layer and a gallium nitride-based semiconductor layer are formed on the intermediate layer.
Patent Document 3 discloses in Example 26 an invention of a laser element where a 3 μm-thick Si-doped n-type GaN buffer layer is formed on a GaN substrate and a stack structure is built on the n-type GaN buffer layer. Incidentally, it is stated that a buffer layer of 300 Å or less formed at a low temperature of about 500° C. may be provided between the n-type GaN buffer layer and the GaN substrate.
However, the surface flatness and optical characteristics of the nitride semiconductor crystal obtained by epitaxial growth on a substrate with the principal surface being a polar plane are insufficient, and more studies on the growth conditions are demanded. In addition, a problem inevitably caused by the selection of a polar plane as the growth substrate is also recognized. For example, there is known a problem that in a quantum well active layer structure (for example, a quantum well active layer structure composed of InGaN/GaN) formed on the c+-plane of a hexagonal system including a c-plane GaN substrate, so-called quantum-confined Stark effect (QCSE) causes reduction in the recombination probability between an electron and a hole and the emission efficiency becomes lower than ideal.
Under these circumstances, an attempt to produce a quantum well active layer structure on a substrate with the principal surface being a nonpolar plane is made, but epitaxial growth on the a-plane, r-plane or m-plane, which are a nonpolar plane of a hexagonal III-V nitride crystal, is difficult, and a high-quality hexagonal III-V nitride semiconductor stack structure is not obtained particularly on the m-plane at present.
For example, Non-Patent Document 1 states that “with respect to the nonpolar m-plane, growth is difficult”, and on the premise of this difficulty, a special crystal growth method is reported, where a GaN substrate with the principal surface being c-plane is worked in a striped fashion to expose the m-plane as a side wall and a nitride semiconductor crystal is epitaxially grown on the striped side wall surface (m-plane).
Also, Non-Patent Document 2 reports that a GaN layer was grown on a ZnO substrate with the principal surface being m-plane by a plasma-assisted MBE (Molecular Beam Epitaxy) method. However, it is stated that the obtained GaN layer was “slate-like”, where the surface unevenness was severe and a single-crystal GaN layer was not obtained.
Furthermore, Patent Document 4 states that in the case of growing a nitride semiconductor crystal on an m-plane substrate, defects are readily generated in the growth initiating interface, and the following technique is described as a technique for reducing the defects. First, a GaN buffer is grown on an m-plane substrate by an H-VPE (Hydride Vapor Phase Epitaxy) method, and the substrate is once taken out of the reacting furnace to form a dielectric mask which is processed in a striped fashion. Then, using the substrate with the formed mask as a new substrate, an epitaxial film is formed by the MOCVD method and flattened through lateral growth (called LEO or ELO growth) from openings of the striped mask.